The present invention is particularly directed to an integrated multiple particle heating agent and the method of induction heating using such agent, and particularly for heating of another medium, forming of high quality bonds to one or more substrates and the like.
Bonding of materials to each other is involved in the manufacture of a great number of products. The very substantial development and use of non-metal elements and surfaces such as plastics, elastomers, glass, paper, and the like has resulted in a significant development in heating processes based on inductive heating principles. If appropriate particles are imbedded within or applied to a non-metallic medium and subjected to an alternating magnetic field of an appropriate frequency, heat is generated within the particles with corresponding thermally conductive heating of the non-metallic medium. The present inventor has over the years developed a particularly unique and practical inductive heating of non-metallic members based on the use of non-metallic particles which are submicron in size. The particles respond to a radio frequency magnetic field to create heat as a result of hysteresis. Other heating systems have employed basically other distinctly different approaches. For example, it is well known that by using larger conductive particles, eddy currents are formed within the metal particles. Because of the resistance of the ferrous metal particles, the eddy currents create heat centers. Although some hysteresis heating may occur in the larger particles, the ohmic loss is the primary and significant heat source.
The non-conductive ferromagnetic particles for hysteresic heating include the submicron particles which have been widely used by the present inventor in the development of plastic bonding processes and devices. The materials are preferably a ferromagnetic oxide, as distinguished from various other conductive magnetic iron materials. Thus, as disclosed for example, the magnetic iron oxide for hysteresic heating are not conductive and do not generate heat as a result of eddy currents. Generally, the particle widely used by the present inventor for commercial application has been a gamma ferric oxide. The particles used for hysteresic heating are generally identified herein as the non-conductive particles.
An early patent showing the non-conductive magnetic iron oxide for bonding through hysteresic heating is shown in U.S. Pat. No. 3,461,014 to James. A number of other patents by the present inventor as well as others assigned to the common assignee with the present application are available based on the pioneering and development of induction bonding based on hysteresis. In contrast, conductive ferrous metal particles of large sizes are used in eddy current heating. The conductive particles are also well known and commercially available and are generally identified herein as ferrous particles. Typical teachings and examples of such material will be found in U.S. Pat. No. 2,393,541 to Kohler and U.S. Pat. No. 3,620,875 to Guglielmo, Sr.
Various means for improving the bonding characteristic have been developed in the prior art. Thus, U.S. Pat. No. 3,941,641, which issued to the present inventor, discloses addition of magnetic metal alloys in a hysteresis heating agent and application of a low or audio frequency field for purposes of agitating the interface of the bonding materials. That patent also notes that the conductive metal alloy particles may also generate some heat as a result of eddy currents and may in fact generate heat somewhat more quickly as well as providing localized mechanical forces, and that mixing of still other particles may vary the flux characteristic to increase the responsiveness of the ferromagnetic particles. The patent teaching however is particularly directed to the hysteresis particle heating in the presence of an audio frequency source plus a radio frequency source. The audio frequency source interacts electromechanically with the large particle parts to agitate the surface. The high or radio frequency source creates the significant and operative heating through the hysteresis effect.
As more fully disclosed in the various patents of the inventor, inductive heating using nonconductive ferromagnetic particles has significant advantages from the standpoint of the frequency required as well as time and quality of bonds. Further, various materials which can only be joined with great difficulty can be appropriately joined with a fusion bond by the use of an appropriate interface bonding element having improved compatibility with the noncompatible plastic substrates. The result, of course is a highly improved product.
Further, eddy current heating with the large ferrous particles requires use of high frequency equipment generally operating in a range of 5 to 30 megahertz. In contrast, the use of the non-conductive ferromagnetic oxide particles with hysteresis operates in a much lower frequency range and generally in a range of 2 to 4 megahertz. Although eddy current heating of particles can be effected at lower frequencies including those in the range of 2 to 4 megahertz, a significant decrease in the heating rate results and is uniformly considered as undesirable and impractical. As the frequency decreases, skin effect phenomenon adversely effects eddy current formation and reduces the creation of heat. Similarly, with eddy current heating, relatively large particles are required to permit formation of circulating current flow in the particles.
In all forms of induction heating, the system and apparatus should be designed to provide a maximum heating rate for any given power or current, and conversely for any given heating rate, the design should provide for minimum power consumption. A high heating rate of course is desired for maximum efficiency of production. Minimal power consumption is of course desired to minimize the direct cost of production. Optimum heating rates and minimum power consumption may however require expensive and specially designed coils. Thus, it is known that increasing the current in a coil in either of the inductive heating processes will result in an increasing heat rate. In hysteresis formulations, the rate increases quite rapidly whereas in eddy current formulations, a more gradual increase is obtained. Apparently as the current and power increases, the hysteresis loop of the non-metallic ferromagnetic oxide and like nonconductive materials becomes significantly more square. This establishes a significant increase in the heating. However, the design requirements for the coils to make use of high current and frequency levels can be extremely difficult to implement.
Notwithstanding the wide usage and development of the various inductive heating processes using particles related to a particular heat generating phenomenon, there is a continuous need for improved processes and materials. Modern day mass production is consistently and continuously concerned with reducing the time and cost of production without reducing the quality of product.